2.42: Bohr's Atomic Model

Look at the people in the picture. Do you see how they are standing on different rungs of the ladder? When you stand on a ladder, you can stand on one rung or another, but you can never stand in between two rungs. A ladder can be used to model parts of an atom. Do you know how? Read on to find out.

Modeling the Atom

The existence of the atom was first demonstrated around 1800 by John Dalton. Then, close to a century went by before J.J. Thomson discovered the first subatomic particle, the negatively charged electron. Because atoms are neutral in charge, Thomson thought that they must consist of a sphere of positive charge with electrons scattered through it. In 1910, Ernest Rutherford showed that this idea was incorrect. He demonstrated that all of the positive charge of an atom is actually concentrated in a tiny central region called the nucleus. Rutherford surmised that electrons move around the nucleus like planets around the sun. Rutherford’s idea of atomic structure was an improvement on Thomson’s model, but it wasn’t the last word. Rutherford focused on the nucleus and didn’t really clarify where the electrons were in the empty space surrounding the nucleus.

The next major advance in atomic history occurred in 1913, when the Danish scientist Niels Bohr published a description of a more detailed model of the atom. His model identified more clearly where electrons could be found. Although later scientists would develop more refined atomic models, Bohr’s model was basically correct and much of it is still accepted today. It is also a very useful model because it explains the properties of different elements. Bohr received the 1922 Nobel prize in physics for his contribution to our understanding of the structure of the atom. You can see a picture of Bohr
below
.

On the Level

As a young man, Bohr worked in Rutherford’s lab in England. Because Rutherford’s model was weak on the position of the electrons, Bohr focused on them. He hypothesized that electrons can move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. He called these fixed distances energy levels, or electron shells. He thought of them as concentric spheres, with the nucleus at the center of each sphere. In other words, the shells consisted of sphere within sphere within sphere. Furthermore, electrons with less energy would be found at lower energy levels, closer to the nucleus. Those with more energy would be found at higher energy levels, farther from the nucleus. Bohr also hypothesized that if an electron absorbed just the right amount of energy, it would jump to the next higher energy level. Conversely, if it lost the same amount of energy, it would jump back to its original energy level. However, an electron could never exist in between two energy levels. These ideas are illustrated in the
Figure
below
.

This is a two-dimensional model of a three-dimensional atom. The concentric circles actually represent concentric spheres.

Q:
How is an atom like a ladder?

A:
Energy levels in an atom are like the rungs of a ladder. Just as you can stand only on the rungs and not in between them, electrons can orbit the nucleus only at fixed distances from the nucleus and not in between them.

Energy by the Spoonful

Bohr’s model of the atom is actually a combination of two different ideas: Rutherford’s atomic model of electrons orbiting the nucleus and German scientist Max Planck’s idea of a quantum, which Planck published in 1901. A
quantum
(plural, quanta) is the minimum amount of energy that can be absorbed or released by matter. It is a discrete, or distinct, amount of energy. If energy were water and you wanted to add it to matter in the form of a drinking glass, you couldn’t simply pour the water continuously into the glass. Instead, you could add it only in small fixed quantities, for example, by the teaspoonful. Bohr reasoned that if electrons can absorb or lose only fixed quantities of energy, then they must vary in their energy by these fixed amounts. Thus, they can occupy only fixed energy levels around the nucleus that correspond to quantum increases in energy.

Q:
The idea that energy is transferred only in discrete units, or quanta, was revolutionary when Max Planck first proposed it in 1901. However, what scientists already knew about matter may have made it easier for them to accept the idea of energy quanta. Can you explain?

A:
Scientists already knew that matter exists in discrete units called atoms. This idea had been demonstrated by John Dalton around 1800. Knowing this may have made it easier for scientists to accept the idea that energy exists in discrete units as well.

Summary

In Bohr’s atomic model, electrons move around the nucleus only at fixed distances from the nucleus based on the amount of energy they have. The fixed distances where electrons may orbit are called energy levels.

Bohr arrived at his model by applying Planck’s idea of energy quanta to Rutherford’s atomic model of electrons orbiting the nucleus.

Vocabulary

quantum
: Minimum amount of energy that can be absorbed or released by matter.

Practice

Read the article at the following URL. Be sure to click through the animation at the bottom of the Web page. Then answer the questions below.